1. Field of the Invention
The present invention relates to a gradient index aspherical lens, and more particularly, to a gradient index aspherical lens suitable for use in a zoom lens system.
2. Description of the Prior Art
In the art of lenses for use in cameras and the like, it has been attempted to effectively correct aberrations by using gradient index lenses to thereby reduce the number of lens elements (Japanese Laid-open Patent Application No. H2-51116). Also, it has been attempted to correct aberrations by using aspherical lenses to thereby reduce the number of lens elements (U.S. Pat. No. 5,327,290).
The gradient index lens having only spherical surfaces is capable of correcting chromatic aberration and Petzval sum by itself. However, since the degree of freedom for aberration correction is insufficient, it is after all impossible to realize each lens unit of a zoom lens system by one lens element. This is because at least two radial gradient index lenses are necessary in order to correct all of the Seidel aberrations simultaneously in gradient index lenses having only spherical surfaces (see "Design of a Gradient-Index Photographic Objective", by L. G. Atkinson et al., Appl. Opt. Vol. 21, No. 6,993 [1982]).
On the other hand, since chromatic aberration and Petzval sum cannot be corrected only by using aspherical surfaces, each lens unit necessarily includes at least two lens elements. Therefore, in order to realize each lens unit of the zoom lens system by one lens element, it is indispensable to use an aspherical surface in the gradient index lens to secure a required degree of freedom.
Japanese Laid-open Patent Application No. H4-63312 discloses a gradient index single lens formed to be aspherical which is used as an objective for an optical disk. However, how the gradient index lens is formed to be aspherical is not described. While some methods have conventionally been known to form a gradient index lens to be aspherical, in order to manufacture an aspherical gradient index lens as disclosed by Japanese Laid-open Patent Application No. H4-63312, it is necessary to use a method such as an aspherical surface manufacturing method to form an aspherical configuration by grinding (by so-called generating) and an aspherical surface manufacturing method by glass molding.
However, the former method presents a problem that it is difficult to reduce the cost of the optical system because of the high cost of the lenses. On the other hand, the latter method presents a problem that it is very difficult to set the optical axis of the aspherical surface and the center of gradient (i.e. the symmetry axis of the gradient) so as to coincide with each other and a problem that it is difficult to obtain a desired gradient index since the gradient configuration is changed by molding. Therefore, it is difficult to manufacture a gradient index aspherical lens by a method to directly process a lens having a gradient index, and even if it can be manufactured, the lens will be rather expensive.